This invention relates to receivers for the Navstar satellite navigation system.
Navstar is a satellite navigation system which is planned to give continuous worldwide all-weather coverage, providing highly accurate, three-dimensional position and velocity information.
The complete Navstar system is planned to consist of 18 satellites arranged in nearly circular orbits with radii of 26,600 km, and an inclination to the earth's equatorial plane of 55 degrees. Each satellite transmits two navigation signals, designated L1 and L2 and centred at 1575 and 1228 MHz respectively.
Both signals convey ranging information by means of modulations which are locked in time to atomic standards. The forms of these modulations (which are known as pseudorandom codes because they appear random, but are nevertheless well defined) are unique to each satellite.
By measuring the phases of the received codes against a clock in the receiver, together with the Doppler shifts of the radio frequency carriers, a user can calculate the range and range rate to a particular satellite by monitoring four satellites (FIG. 1). By decoding data about their motions which are also modulated on to the transmitted signals, the user may solve equations (FIG. 2) to determine his three-dimensional position and velocity and also apply corrections to his clock, making it conform to satellite time.
Alternatively, if he is constrained to move on the surface of the earth or is at known altitude, he may make two-dimensional measurements using three satellites. The software controlling the receiver must choose from the satellites in view the subset which gives the most favourable geometry for the navigational calculations.
Two pseudorandom codes are in fact transmitted by each satellite. The first of these is used to aid acquisition of the satellite signals and to provide coarse navigation, and hence is called the Coarse/Acquisition (C/A) code. The second has a 10-times higher modulation rate which yields the full navigational accuracy of the system, and is designated the Precision (P) code.
A basic Navstar receiver typically contains a low-noise amplifier and down-converter to a convenient IF, followed by one or more code and carrier tracking channels, each capable of tracking the transmissions from any satellite. There is also associated range and range-rate measurement circuits.
The purpose of the code tracking loop is to keep a code generator in the receiver in step with a received pseudorandom sequence, and hence provide information on the range to the satellite being tracked.
To obtain a position and velocity estimation, a receiver must be locked to the transmissions from a number of satellites. Consider the case of a complete three-dimensional estimation for which the required number is four, as depicted in FIG. 1. Four measurements of "pseudo-range" are made by locking code tracking loops to the received signals and then timing the occurrence of certain states of the code generators within the loops with the aid of the receiver's clock. The measurements are of "pseudo-range" rather than true range because of the (as yet) undetermined receiver clock offset.
Similarly, by measuring the frequencies of the carrier tracking loop voltage-controlled oscillators over gating times determined by the receiver clock, four measurements of "pseudo-range rate" are obtained. These are in error from the true range rates because of the clock's frequency error. All these measurements, together with data from each satellite which provides information about satellite motion, then enable a navigational solution to be obtained. This relies on the fact that four observations are required to solve for four unknowns.